Manufacture of photo-electric cathodes



A ril 18, 1933. N. R. CAMPBELL 1,904,895

MANUFACTURE OF PHQTO ELECTRIC CATHODES Filed Dec. 16, 1950 Inuem or,

Patented Apr. 18, 1933 mmnmcrunn or rirorornrncriuci canteens} {15,1

Application ijlle d December 16,1930, Serial no. 502,659, the iqrest iarifei iiefiuafia, res

r This invention relates to the manufacture of photoelectric cathodes of the type produced by the deposition of a highly electropositive metal on a support coated with-a compound with gwhich thatmetal reacts chemically; Photoelectriccathodes are known consist ing of a thin'film of an alkali or alkaline earth metal deposited on an oxidized surface of'another metal, such ascopper or silver. In one known process of preparing such cathodes, there are three distinct stages In the first stage the underlying metal is oxidized; in the second, the alkali'inetal isintroduced into thetube containing the cathode; in the f third, the cathode, and usually the whole of the' tube containing it, are baked. During this third stage, someireaction'take's, place between the alkali metalan'd its support; and, if there is anyalkali metal remaining in';ex-

cessafter this reaction, it is driven-from the cathode, and usually out of the tube;

The properties of theica'thode' produced by this process depend in some measure on the quantity of the electropositive metal introduced during-the second stage. The facts have" been explored most thoroughly" when the alkali metal is caesium and the under lying'metal silver; it. will be 'co'nvenientlto describe the invention with reference to'this example. If the quantity isless' than a certain limit, depending on the" area of the cathode as" 'well ason other factors, the surface of the cathode becomes relatively dark in the third stage and no excess "of caesium appears; this state of the cathode will be called A. If the quantity is greater than this. limit, thecolour-of the cathode is considerably lighter afterithe third stage and excess caesium appears; this state will be called 13 In state A the photoelectric emission increases in general with the amount of caesium introduced; but in state B it may decrease with the amount introduced. 'There is thus an optimum amount of caesium that should be introduced in the second stage, corresponding roughly to the limits between'the amounts that lead" to states Aand B.

But I have 'found that thephotoelectric emission depends not only on the amount of NORMAN 301mm, ailment, on warronnfnivenmig giqfidgfi mg; um p i nmcmnrc company umrnngog "LONDOEEENGLAVND j caesium introduced, 'but also on thei rate at Which it is introducedgand on thetemperature h of the cathode support during its introduc tion. 1' The best resultsareobtained by com bini ng the-second and" third; stages,-: so that the caesium" is introduced slowly while the cathode support is hot. A convenient "way ofefi'ectingathis is when the caesiumin a side vessel connected to'the tube througha constriction and to maintain both the: tube and the caesium "containerat a temperature of about 200 C., sothatf the caesiumvapour distils 'slowly' through; the constriction into the tube containingthe hot'cathode.

According to the first fe'ature of the inven e 11,; in the manufacture ,of photoelectric cathodes offtheatype described, the cathode support on which the cathode istobe formed 1 ismain'tained at a temperatureconsiderably above 509 iCwhile-the viapour of the electropositive metal is brought into contact with it inlet-stream considerably 'slower than would a be obtained if that metal were 5 maintained at the temperatureofthe'cathode"andfin its a V immediate neighbourhood in a vac um." I g In carrying;- out the process accordingj'to' the invention it'isn'ecessary to step the supply of caesium whenthe optimum 'la'mount has been introducedfi There aretwo' obvious ways of doing' lthis, 1 One is tolilnit the.

optimum amount; another is to stop the process at intervals and test the photoelectric emissiong-"Neither of these 'is convenient. The first is inconvenient because thmmma'te control of small amounts 'oficaesium' 'is difli-v V cult, and also because the} exact amount Zof of caesium "has been'i'ntroduced; when the 80, 7 ,amount ofcaesium originally"present to the i l limit between states A and B has been small,

'tinued with the caesiumflcontainer removed,

reached, the thermionic emission decreases suddenly. Accordingly in order to determine when the optimum amount has been introduced, it is only necessary to watch the change of thermionic emission; as soon as the gradual rise stops and is followed by a rapid fall, the oven is removed and the supply of caesium stopped by sealing off .the

caesium container at the constrictions Im- 2 mediately after this operation-is performed, the photoelectric emission is comparatively If the baking'at 200*C.is new conthe thermionic and the photoelectric emissions-increase vonce more and the latter gradually attains a-n aximum; not greatly altered by further baking. The process can therefore be completed by bakingfor'a period known to be longenough for this maximum to be attained. No further watching is 're- 7 from the ca-thodein that it .increasesrapidly with the temperaturefand can be saturated by a sufiicientlyela'rge potential. But .I do not commitimys'elf to the view that it is a truethermionic current dependent only on the temperature and surface of the cathode; it is possible that it'consists, wholly or in part, of ions or electrons generated by the chemicalreaction proceeding} A i One'example of the. processaccording to the invention will now be described-with reference-tothe accompanying drawing, The

photoelectric tube is of an ordinary type,con-' sisting of a cylindrical' glassvessel a 40 millimetres in diametenand 8O millimetres long, on theinside wall -of, which silver has been deposited chemically, leaving a window b'for the entry of light and a bare portion a for insulation around the anode lead. The cathode lead is awire cl-sealed through the wall of the vessel and making contact with the silver coating; the anode is a wire framee or a rod supported axially in the vesselby its lead f vwhich passes out through one end. Alternatively the cathodemay be a silver plate supported in thje tube and; the anode a gauze surrounding it; indeed any of the familiar types of tube may be used. The vessel isprovided with two side tubes, one of which g leads to the exhaust pump and theother 72,; is a tube parallel to the axis of the vessel anda few millimetres distant from it. The bottom of this second tube It is sealed usual manner; during 'this' process the bottom of the tube containing caesium azide should project from the oven'and should not be allowed to reach a temperature much exceeding 300 C! The silver coating is then super ficially oxidized. The oxidation may be effected conveniently ina'knownmanner by filling the vessel with oxygen toapressure of one millimetre and passing an electric glow discharge through the vessel with the silver coating as cathode. The coating changes colour as it oxidizes, and the oxidation may be stopped at an appropriate stage when the right colour, determined by trial, has been attained. Considerable latitude is permissible; but I have found a deep blue colour to be suitable. The oxygen'is pumped out and the; caesium azide decomposed by heating the tube, so that caesium 1S liberated. ,With the exhaust pump always in action the oven is lowered again at a'temperature of about .200C.; it is dangerous to exceed 220 0., but lower temperatures than 200 C. merely prolong the process. The electrodes of the tube are connected in. series with. a galvanometer or microammeter and a sourceof about 20 volts,the silver coating being negative. After a minute'or I two a current is indicated, and increases till it reaches a maximum which isusually in theneighbourhood of 5:0 microamperes after a time of the order of 10 minutes, but depending greatly on the size of the constriction through, which the caesium vapour enters. After the maximum thecurrentdecreases very rapidly; it may fall to a small fraction of; a'microampere; orit may fall and then rise. again rapidly, being new unsaturated and obeying Ohms law. This unsaturated current is carried by a film of excess caesium bridging the insulation. -Again the current having reached a first maximum may fall slowly and slightly and then rise slowly to a second and higher maximum from which the rapid fall takes place. The'changes of the current may vary considerably; but the rapid fall, veither to-almost zero or to a current that is definitely unsaturated, can always be detected. As soon as this fall has certainly started, the temperature .is raised and the caesium tube sealed off," The term perature is then lowered again for about half A an hourto remove the excess caesium. -Finallythe tube may be filled with a rare gas,

sensitized and sealed off.

when the said space current diminishes rapidly.

2. In the manufacture/of a photoelectric cathode, the process which comprises forming on the'electrode that is to become the cathode a surface layer. consisting of a compound that reacts with the electropositive metal that is to be introduced, heating said electrode to a temperature exceeding 50 0., producing a slow stream of the vapour of the electropositive metal and bringing the said vapour into contact with the said surface layer while the'said electrode is maintained at said temperature-exceeding 50 C. 3..In the manufacture of a photoelectric cathode the process which comprises taking a tube and forming on the electrode that is producing a slow stream of the vapourof I said vapour into contact with the said surface the electropositive metal and bringing thev layer while the said electrodefis maintained at'said temperature exceeding50 C. measuring the space current of thevphotoelectric tube during the passage of the said stream of vapour and stopping. the stream when the said s ace current diminishes ra idly. c' 1 ORMAN ROBERT CAL FPBELL.

to become the cathode a surface layer con- 1 sisting of a compound that reacts with the electropositive metal that is to be introduced,- said tube containing the said electrode being connected to a vessel containing the electro-' positive metal through a tube of small cross section, then heating both the tube and the I vessel to a temperature exceeding 50 (3.; and maintaining said temperature so that a slow stream of the vapour of the electropositive, metal passes from said vessel to said tube'and V is brought into contact with saidsurface layer. i

4. In the manufacture 'of a'photoelectrics cathode the process which comprises forming on the electrode that is to become the cathode a surface layer consisting of a compound 7 that reacts with the electropositive metal that is to be introduced, heating said electrode to a temperature exceeding 50 C., producing a slow stream of the vapour of the electropositive metal andbringing the said Vapour into contact with the said surface layer while the said electrode is maintained at said temperature exceeding 50 0., and stopping the stream when the optimum amount of vapour of the electropositive metal has been introduced into the neighborhood of thesaid support.

5. In the manufacture of a photoelectric cathode, the process which comprises forming on the electrode that is to become the r cathode a surface layer consisting of a com pound that reacts with the electropositive metal that is to be introduced, heating said electrode to a temperature exceeding 50 C.', 

